[postgis-devel] EXTENSION upgrade scripts are now symlinks

[Sandro Santilli]

I've changed install scripts to install a single
real file for extension upgrades, named:

  postgis--ANY--${version}.sql

and turn all the others (one for each upgradeable
version) to symlinks to that single file.

Please test it on your systems, see if you like it.
Once everyone is happy it'll need to be done for all
extensions.

--strk;